Colloidal semiconductor nanocrystals, or quantum dots, have been the focus of a lot of research. Colloidal quantum dots, hereto within referred to as quantum dots or nanocrystals, are easier to manufacture in volume than self-assembled quantum dots. Colloidal quantum dots can be used in biological applications since they are dispersed in a solvent. Additionally, the potential for low cost deposition processes make colloidal quantum dots attractive for light emitting devices, such as LEDs, as well as other electronic devices, such as, solar cells, lasers, and quantum computing devices. While potentially broader in their applicability than self-assembled quantum dots, colloidal quantum dots do have some attributes that are comparatively lacking. For example, self-assembled quantum dots exhibit relatively short radiative lifetimes, on the order of 1 ns, while colloidal quantum dots typically have radiative lifetimes on the order of 20-200 ns. Colloidal quantum dots also exhibit blinking, characterized by a severe intermittency in emission, while self-assembled quantum dots do not have this characteristic.
Of particular interest are II-VI semiconductor nanocrystals. These nanocrystals have size-tunable luminescence emission spanning the entire visible spectrum. In photoluminescent applications, a single light source can be used for simultaneous excitation of different-sized dots, and their emission wavelength can be continuously tuned by changing the particle size. Since they are also able to be conjugated to biomolecules, such as, proteins or nucleic acids, this photoluminescence property makes them an attractive alternative for organic fluorescent dyes classically used in biomedical applications. Additionally, the tunable nature of the emission makes quantum dots well suited for full color display applications and lighting. As a result of their well-established high-temperature organometallic synthetic methods (Murray et al, J. Am. Chem. Soc. 115, 8706-8715 1993) and their size-tunable photoluminescence (PL) across the visible spectrum, CdSe nanocrystals have become the most extensively investigated quantum dots (QD).
As noted by Holing et al (J. Am. Chem. Soc. 126 1324-1325 (2004)), colloidal semiconductor quantum dots are also brighter and far more photostable than organic dyes, making them particularly interesting for biological applications. It also has been reported in the open literature that surface passivation of quantum dots with a semiconductor layer having a wider band gap or with polymers improves the optical properties of quantum dots, such as, quantum yield and photobleaching. The blinking behavior of quantum dots, however, is generally considered an intrinsic limitation that is difficult to overcome. This is unfortunate because growing applications in spectroscopy of single biological molecules and quantum information processing using single-photon sources could greatly benefit from long-lasting and nonblinking single-molecule emitters. For instance, in a recent application of single-dot imaging, the tracking of membrane receptors was interrupted frequently due to the stroboscopic nature of recording. Blinking can also reduce the brightness in ensemble imaging via signal saturation. Furthermore, blinking limits the use of colloidal quantum dots in luminescent applications such as single molecule LEDs.
A few groups have been working on solutions to the colloidal quantum dot blinking problem, especially for biological applications. It was found in 2004 by Holing et al (Hohng et al., J. Am. Chem. So. 126, 1324-1325 (2004)) that quantum dot blinking could be suppressed by passivating the QD surface with thiol moieties. The experiments by Hohng et al were conducted with CdSe/ZnS quantum dots that showed inherent blinking behavior. Larson et al studied encapsulating the QDs within an amphiphilic polymer (Larson, et al., Science 300, 1434-1435, 2003), using water soluble CdSe/ZnS QDs. The results of Holing et al and Larson et al do not solve the intrinsic problems resulting in blinking dots, they only control the environment at the surface of the dots in order to mitigate the problem. Both approaches are only useful in end applications that remain in solution and allow particular surface passivations.
In addition to the problem of blinking, colloidal quantum dots suffer from increased radiative lifetimes as compared with their self-assembled counterparts. Radiative lifetime is defined as the reciprocal of the first-order rate constant for the radiative step, or the sum of these rate constants if there is more than one such step (IUPAC Compendium of Chemical Terminology, 2nd Edition (1997)). Short radiative lifetimes are desirable in order to compete successfully with non-radiative recombination events, such as, Forster energy transfer.
Although quantum dots containing CdSe cores are arguably the most studied and best understood of the quantum dots, some researchers are looking at more complex quantum dots with ternary rather than binary compositions. U.S. Pat. No. 7,056,471 by Han et al discloses processes and uses of ternary and quaternary nanocrystals (quantum dots). The nanocrystals described by Han et al are not core/shell quantum dots, rather they are homogeneously alloyed nanocrystals (also referred to as nanoalloys). Although Han et al do not address the issue of blinking in their disclosure, Stefani et al us use nanoalloy dots made by the disclosed process for a study of photoluminescence blinking (Stefani et al, New Journal of Physics 7, 197 (2005)). Stefani et al found that monocrystalline Zn0.42Cd0.58Se QDs with an average diameter of 6.2 nm exhibited photoluminescence blinking. Although Stefani et al do not discuss the radiative lifetimes of their ternary nanocrystals, Lee et al have studied colloidal ternary ZnCdSe semiconductor nanorods (Lee et al, Journal of Chemical Physics 125, 164711 (2006)). Lee et al found that the ternary nanorods exhibit radiative lifetimes slightly longer than comparable CdSe/ZnSe core/shell nanorods. The CdSe/ZnSe nanorods had lifetimes around 173 ns, while the shortest lifetime for the ternary rods was observed to be 277 ns.
While researches in biological fields are looking to quantum dots to replace organic fluorescent dyes, quantum dots also hold promise for use in electronic devices. Research is ongoing into incorporating quantum dots into photovoltaics, solid-state lighting (mainly as quantum dot phosphors), electroluminescent displays as well as quantum computing devices. Semiconductor light emitting diode (LED) devices have been made since the early 1960s and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers including the LEDs are based on crystalline semiconductor materials that require ultra-high vacuum techniques for their growth, such as, metal organic chemical vapor deposition. In addition, the layers typically need to be grown on nearly lattice-matched substrates in order to form defect-free layers. These crystalline-based inorganic LEDs have the advantages of high brightness (due to layers with high conductivities), long lifetimes, good environmental stability, and good external quantum efficiencies. The usage of crystalline semiconductor layers that results in all of these advantages, also leads to a number of disadvantages. The dominant ones are high manufacturing costs, difficulty in combining multi-color output from the same chip, and the need for high cost and rigid substrates.
In the mid 1980s, organic light emitting diodes (OLED) were invented (Tang et al, Appl. Phys. Lett. 51, 913 (1987)) based on the usage of small molecular weight molecules. In the early 1990s, polymeric LEDs were invented (Burroughes et al., Nature 347, 539 (1990)). In the ensuing 15 years organic based LED displays have been brought out into the marketplace and there has been great improvements in device lifetime, efficiency, and brightness. For example, devices containing phosphorescent emitters have external quantum efficiencies as high as 19%; whereas, device lifetimes are routinely reported at many tens of thousands of hours. In comparison to crystalline-based inorganic LEDs, OLEDs have much reduced brightness (mainly due to small carrier mobilities), shorter lifetimes, and require expensive encapsulation for device operation. On the other hand, OLEDs enjoy the benefits of potentially lower manufacturing cost, the ability to emit multi-colors from the same device, and the promise of flexible displays if the encapsulation issues can be resolved.
To improve the performance of OLEDs, in the later 1990s OLED devices containing mixed emitters of organics and quantum dots were introduced (Matoussi et al., J. Appl. Phys. 83, 7965 (1998)). The virtue of adding quantum dots to the emitter layers is that the color gamut of the device could be enhanced; red, green, and blue emission could be obtained by simply varying the quantum dot particle size; and the manufacturing cost could be reduced. Because of problems, such as, aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., J. Appl. Phys. 93, 3509 (2003)). The poor efficiency was attributed to the insulating nature of the quantum dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a monolayer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the organic molecules (electron-hole recombination occurs on the organic molecules). Regardless of any future improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) that were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection into the quantum dots. In addition, the remainder of the structure is costly to manufacture, due to the usage of electron and hole semiconducting layers grown by high vacuum techniques, and the usage of sapphire substrates.
Accordingly, it would be highly beneficial to construct an all inorganic LED based on quantum dot emitters which was formed by low cost deposition techniques and whose individual layers showed good conductivity performance. The resulting LED would combine many of the desired attributes of crystalline LEDs with organic LEDs.
For solid state lighting applications, the fastest route to high efficiency white LEDs is to combine either blue, violet, or near UV LEDs with appropriate phosphors. Replacing traditional optically pumped phosphors with quantum dot phosphors has many advantages, such as, greatly reduced scattering, ease of color tuning, improved color rendering index (CRI), lower cost deposition process, and broader wavelength spectrum for optical pumping. Despite these advantages, quantum dot phosphors have not been introduced into the marketplace due to some major shortcomings; such as, poor temperature stability and insufficient (10-30%) quantum yields for phosphor films with high quantum dot packing densities. In order to raise the quantum yield, many workers have lowered the packing density by incorporating appropriate filler (e.g., polymers or epoxies) with the quantum dots. The disadvantage of this approach is that the resulting quantum dot phosphor films are unacceptably thick (1 mm), as compared to the desired thickness of 10 μm. As has been discussed by Achermann et al (Achermann et al., Nano Lett 6, 1396 (2006)), reduced quantum yields for dense films is mainly the result of inter-nanoparticle interactions that lead to exciton transfer (Forster energy transfer) from emitting quantum dots to non-emitting quantum dots. Since the Forster energy transfer rate decreases rapidly with distance, d, as 1/d6, a way to minimize this effect is to form low density films (with the aforementioned problems). A more desirable approach would be to decrease the radiative lifetime of the quantum dot emitters in order to compete more effectively with the Forster energy process, while enabling dense films of quantum dot phosphors. More specifically, the Forster energy transfer time for drop cast films of quantum dots has been experimentally measured to be on the nanosecond time scale (Achermann et al., J. Phys. Chem. B107, 13782 (2003)).
To date, optoelectronic devices or biological studies have not had colloidal quantum dots available that are inherently non-blinking or that have short radiative lifetimes. Previous methods to create non-blinking dots are application dependent and not universally applicable across the technical disciplines utilizing quantum dots. While self-assembled quantum dots exhibit short radiative lifetimes, there are no reports of colloidal quantum dots exhibiting similar performance. Therefore, there is a need for colloidal quantum dots with inherent non-blinking behavior for use in biological and electronics applications. Additionally, there is a need for quantum dots with short radiative lifetimes that could be used in biological and optoelectronics applications.